div.top{margin:50px;padding:20px;width:200px;height:200px;border:2px dashed #ebbcbe;}
div.top div{width:100px;height:100px;padding:10px;color:white;}
div.a{background:red;}
div.b{background:green;position:absolute;top:100px;left:100px;z-index:999;}